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Tuesday, 1 July 2014

Mapping spacetime around supermassive black holes

Black holes come in many sizes ranging from tens to millions, or even billions, of solar masses. Their incredible size means they exert immense gravitational power over other objects, and can even warp space-time to such a degree that they behave like lenses and actually bend light around them – a process known as gravitational lensing. In many cases a large black hole will acquire another incredibly dense friend, for example a small black hole or a neutron star, which will orbit the central black hole whilst slowly spiraling into it. These physical systems are known as Extreme Mass Ratio Inspirals (EMRI's), called as such because of the vast mass difference between the two objects.

Physicists often consider space and time as a single continuum, called spacetime, which consists of the 'usual' three dimensions (up/down, left/right and forwards/backwards) plus time as a 'fourth' dimension. Spacetime is bent by anything with mass - an effect we see as gravity. Image credit: Wikimedia commons

Einstein’s famous theory of general relativity states that any mass will bend spacetime. Black holes, because they are so incredibly dense, will stretch and curve space-time to a much greater degree than our planet ever could. However something relatively tiny, like the Earth, still has an effect. For EMRI's, you can think of this as being like a bowling ball placed on to a taut sheet - the bowling ball will sink causing the sheet to stretch. If you place a marble onto the same sheet, it will also sink a little bit into the sheet because it has its own weight, but the bowling ball makes a much larger dip than the marble.

But getting out sheets, marbles and bowling balls isn’t a very accurate way of modelling these systems – so how is it done? I spoke to Dr Sarp Akcay, a postdoctoral fellow at the University of Southampton and an expert at creating models simulating the orbits of EMRI's.

“At first, one looks at the simplest possible picture - an approximation to the physical reality. So, one sets the mass of the small object to zero. Such zero-mass objects are called test masses and trace out curves called ‘geodesics’ in the space-time of the larger black hole. The trajectory of each geodesic is determined completely by the curvature of space-time due to the large black hole.”

A "geodesic" is the shortest distance between two points on a curved surface. Image credit: wikimedia commons

A geodesic is a line which makes the shortest possible path between two points on a curved surface, in this case the curved surface is the space-time in which the central black hole sits. Taking the simplest picture first makes it easier for astronomers to understand exactly what’s going on. Once this is clear, they start to add in more parameters, like replacing the zero-mass with a small mass.

The smaller object travels round in its orbit, slowly losing energy which causes its orbit to gradually get smaller and smaller, until eventually it falls right into the central black hole. In this model, the way it loses its energy is by ‘gravitational radiation’ - gravitational energy transported by gravitational waves.

“Physically, the small mass creates a back-reaction to its motion - its own small gravitational field interacts with the large gravitational field of the warped space-time. This interaction between the two objects causes gravitational radiation to be emitted.”

These strange waves are depicted as undulations in space-time, not unlike ripples across the surface of a pond. Despite being predicted to exist in 1915 by Einstein, this bizarre phenomenon has not yet been directly observed - which leads us to the question: How can we build something to detect an undiscovered signal?

It’s all to do with the model. A more accurate model will predict a certain frequency of the radiation being emitted and so scientists can use this information to fine-tune the instruments on future space-based detectors to pick up this frequency. Any gravitational observatory pointed into space is going to pick up hundreds of signals at once, but if astronomers know exactly what kind of signal they are looking for they can untangle the mess of noises to find the signal which matches closest with predictions, and - voila! Gravitational radiation containing reams of valuable data is available for scrutiny.

“We would get months to years worth of data, which then will enable us to extract information about the central massive black hole. This is known as "mapping the spacetime" around the central black hole and would enable us to measure both the mass and the spin of the black hole to better than 1% accuracy.”

This artist’s impression shows two massive objects rotating around a central point. Their interacting gravitational fields wobble the space-time around it sending waves spiralling outwards. These waves are what astronomers hope to detect. Image credit: NASA

Modelling these kinds of systems is all very cool, and the implications of being able to predict behaviours and characteristics, not to mention discovering gravitational radiation, could change the way we look at the night sky. “We think such orbits may exist in the centres of galaxies where black holes with masses of millions to billions of suns reside.” So the more we can find out about such systems, the more we can explore the thousands of galaxies, including our own.

Not only will these kinds of observations reveal more about the nature of our galaxy, but about black holes as well. These infinitely dense singularities are shrouded in secrecy and yet they power the brightest objects in the universe - Quasars. Astronomers believe that black holes can exist as two main types. One such type is called the Kerr-type black hole - in short a black hole which rotates. By observing EMRI’s, scientists hope to answer the long-standing question known as ‘The Kerr Conjecture’ - whether or not all black holes in the universe are of the spinning Kerr type.

Collecting this amount of data about black holes is invaluable, not only because it is thought that one lies at the very centre of our own galaxy, but also because by revealing the nature of black holes and spacetime we can uncover more about the fundamental laws which govern the universe. Even gravity, which we’ve all experienced but cannot fully explain, might expose its true nature through observations of this kind.